[LLVMdev] Pointer aliasing

[Duncan Sands]

Hi Umesh, when the compiler can see that the storage locations do not overlap
there is no problem, for example when storing to variables located on the stack
like in your example.  The tricky case is when the compiler can't deduce from
what it can see that the storage locations don't alias, so needs to be helped
for example by using a restrict qualifier.

Ciao, Duncan.

> I'm newbie to LLVM and Clang ,But has experience on compiler optimization  and  VM .
> Everyone talking about the  LLVM in my organisation so thought of peeking into
> it  and where this discussion is  stalled me ...
>
> so  i  tried to simulate the problem ,which is discussed here .
> vi sample.c
>
> double f(double** p )
> {
>    double a,b,c;
>    double * x = &a;
>    double * y = &b;
>    double * z = &c;
>
>    *x = 1;
>    *y = *x + 2;
>    *z = *x + 3;
>
>    return *x+*y+*z;
> }
>
> double ff(double** p )
> {
>    double a,b,c;
>    double * x = &a;
>    double * y = &b;
>    double * z = &c;
>
>    *x = 1;
>    *y = *x + 2;
>    *z = *x + 3;
>
>    return *x+*y+*z;
> }
>
> compiled the sample.c .i.e  clang sample.c -S  -O3 -emit-llvm
> cat sample.s
>
> target datalayout =
> "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S32"
> target triple = "i386-pc-cygwin"
>
> define double @f(double** nocapture %p) nounwind readnone {
>    ret double 8.000000e+00
> }
>
> define double @ff(double** nocapture %p) nounwind readnone {
>    ret double 8.000000e+00
> }
>
> Boom ...BasicAA or TBAA is  doing what it suppose to do :) for restrict
> qualifier too..
>
> Any lights on this ???
>
> FYI , $ lli --version
> Low Level Virtual Machine (http://llvm.org/):
>   llvm version 3.0
>   Optimized build.
>   Built Jan 24 2012 (05:48:10).
>   Host: i386-pc-cygwin
>   Host CPU: penryn
>
> Thanks
> ~Umesh
>
>
> On Fri, Jan 27, 2012 at 12:37 AM, Dan Gohman <gohman at apple.com
> <mailto:gohman at apple.com>> wrote:
>
>     On Jan 24, 2012, at 8:58 PM, Brent Walker wrote:
>
>      > Thank you for your reply.  The compromise you describe below, is it a
>      > compromise in the LLVM back end or in clang?  I run into this while
>      > building a compiler for a small DSL language for which I generate
>      > functions that receive a context from which they extract a bunch of
>      > pointers to doubles from which inputs are passed to the function (I
>      > just used C/clang in my examples to illustrate the problem).
>
>     For restrict-style alias information, the limitation is in LLVM.
>
>     Since you're not using clang, you might be able to use a custom TBAA
>     type tree instead. TBAA works differently from restrict; it applies
>     to loads and stores, rather than to pointers. But if you can frame
>     your aliasing property as a type-oriented property, saying for
>     example that each array element is a pointer to a distinct type, it
>     may suffice.
>
>     Dan
>
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